mooring bridon

5/12/2018 Mooring Bridon - slidepdf.com

http://slidepdf.com/reader/full/mooring-bridon 1/14

BRIDON

INTERNATIONAL

INNOVATIONS IN MOORING CABLE SOLUTIONS

Author: Steve Calverley

Bridon International Ltd, Doncaster, UK

Abstract

Tried and tested steel cable products offer a mooring solution with long term experience andperformance data. The suspended weight of steel mooring components presents a challenge as

floating exploration & production facilities move into increasing water depth raising the issue of

perceived limits for steel cables. Therefore Bridon continues to actively address solutions for ultra

deep water with a focus on increased strength, reduced weight and improved endurance.

Advances in mooring system design have developed mooring arrangements, using proven

technology steel components, which are both technically and economically suitable for 2000 metres

(6500 feet) water depth. The potential for improvements to strength to weight ratio suggests that theuse of steel products in increasingly deep-water locations can be achieved. Having gained

experience through consultancy and design roles during the supply of 52 permanent mooring

projects, supporting evidence established through numerous research programs and theoretical

analysis, Bridon will continually challenge the perceived limiting parameters.

This paper will discuss the recent improvements in strength to offer a 25% lighter weight solution than

that defined within DNV Certification Note 2.5, dramatically increasing the depths in which steel

product can economically be used.

Current usage limitations of polyester mooring ropes results in the need for connection to steel

mooring line segments. Understanding the interactions of these differing components and

development of products to service this application is critical. This paper addresses the future of ultra-

deep mooring systems through consideration of the complementary roles of steel and fibre mooring

ropes.

Following the recent MMS approval of the use of FPSOs in the Gulf of Mexico s further expandingmarket demand, Bridon aims to continue to support the requirements of all oil explorations and

extraction applications with a critical focus on the demanding mooring system application.



BRIDON

INTERNATIONAL

1.0 Introduction

Demands on exploration and production are driving the installation of facilities in ever increasing

water depths. Conventional catenary moorings are currently planned for utilisation in depths of

around 2000 metres (6500 feet). In deep water the extended mooring cable lengths result in greater

self weights which influence both operational and installation loads.

Through the development of products with a greater strength to weight ratio, both system and

deployment loads can be minimised. Continued improvements to strength suggest that the use of

technically proven steel products in increasingly deep water locations will remain viable.

2.0 Deep Water Application of Steel Mooring Systems

In catenary systems (figure 1), the floating structure moves laterally in response to environmental

loads. The overall compliance and hence station keeping performance is determined by the water

depth, the weight of the mooring line and the mean tension. In depths beyond 1000 metres, the

vertical load component can become significant due to a fundamental feature of the catenary

mooring system weight (Firth 1997).

Figure 1 — Conventional Catenary Mooring

Semi taut and inverted catenary mooring arrangements (figure 2) introduce buoyancy to the mooring

system. The result being a potential 40% reduction in resultant turret force and 25% reduction in

resultant anchor leg force (Blair et al 1995) when compared to an equivalent conventional catenary



BRIDON

INTERNATIONAL

Figure 2 — Semi Taut Mooring

It is apparent the continued use of steel mooring line components remains possible with increasing

water depth maintaining the confidence provided by proven technology. Nevertheless, continuing

improvements in strength to weight ratio not only support the extension to useful depth range, but

offer a more cost effective solution — reductions in mooring system loads can be utilised through

maximisation of topside equipment.

3.0 Technological Advances

Increased strength or conversely reduced cable self weight has been achieved through advancing

manufacturing techniques, availability of specialised materials and developments in engineering

design.

Figure 3 — Spiral Strand Mooring Cable Cross



BRIDON

INTERNATIONAL

3.1.2 Feed Material

In wire drawing, the quality of the finished product is governed by the quality and properties of raw

materials. Considering the metallurgical properties required for cable manufacture and considering

the expense and availability of exotic materials, the options for feed materials are limited to

developments in micro-alloyed, high carbon / high silicon steels. Through careful control of the

chemical composition and lead patenting conditions in the manufacturing process a feed material of

homogeneous structure is achieved from which the final strength can be generated.

Bridon has recently completed a development program to meet the challenge of manufacturing a10%

higher strength final hot dip galvanised (A-class) wire than that utilised in the Bridon SPR2 plus range

of spiral strands, which maintains good ductile properties and is suitable for spinning into finished

cable. This wire product forms the essential building block of the Bridon Xtreme Spiral Strand range.

The feed material for Xtreme is a high silicon steel which has been developed in conjunction with our

steel supplier, Corus and is the result of a continuing partnership in steel development which has

given a continual improvement in final wire tensile strength.

3.1.1 Wire Drawing Process

The finished wire ultimate tensile strength (UTS) is achieved through the combination of the number

and sequence of drawing dies and the reduction in diameter achieved by each.

Maximising cold working of the wire generates maximum UTS and in general this means that the

smaller the finished wire diameter, the greater the potential tensile strength. Processing in this

manner requires a high level of control to prevent loss of other properties such as ductility and

surface finish.

Improvements in strand or rope breaking load are achieved through utilising a greater number of

smaller diameter higher tensile wires. Mooring industry standards do not specify a minimum wire

diameter. However, wire diameter selection is a balance between tensile strength, constructional

balance and corrosion resistance. Firstly, availability of galvanic protection is proportional to the

diameter of the wire. Secondly, the increased ratio of surface area to cross sectional (load bearing)

area demonstrates the greater corrosion effect on a smaller diameter wire.



BRIDON

INTERNATIONAL

4" 4.25" 4.5" 4.75" 5" 5.25" 5.5" 5.75" 6"

5000

7000

9000

11000

13000

15000

17000

19000

21000

23000

25000

M B L ( k N )

Diameter

DNV CN 2.5

Bridon SPR2

Bridon SPR2plus

Bridon Xtreme

Figure 4 — Spiral Strand Strength Progression

Xtreme has been extensively tested throughout the manufacturing process i.e. rod and wire

properties and finally minimum breaking load and fatigue testing. Two full scale fatigue tests have

been conducted on 100mm diameter Xtreme spiral strand with an MBL of 1100 tonnes (10,791kN) —

Appendix 1.

Current developments in six strand ropes for drilling rig operations have achieved breaking loads 30%

higher than the API 9A minimum or conversely a 30% lighter, smaller rope. It is expected that the wire

developments recently attained can be utilised to similar effect for drawn galvanised six strand wire

ropes.



BRIDON

INTERNATIONAL

3.3 Cable Terminations

It is essential the termination strength at least matches the cable. Advances have been made in

cable termination design with higher strength materials enabling higher strength products to match

cable strength developments. Closed sockets have been developed facilitating the direct connection

between cable and chain removing the need for additional connector plates.

Figure 6 — Cable Termination Options

Future work is envisaged based on increased material strength and geometry changes to allow easeof connection to other more standard mooring system components.

4.0 Mooring Systems Weight Savings

Lighter weight products will allow designers to increase the water depth in which steel products are

economically and logistically viable or allow more cost effective development of shallower water

locations.

The typical overall submerged weight saving achievable for a twelve leg spread moored system of

2000 metre mooring cables each of 1000 tonne breaking load for a typical for a West Africa FPSO

system, can be estimated at 100 tonne reduction in static load. Similar comparison for a Gulf of

Mexico typical Spar system with nine 1200 metre mooring cables each of 1600 tonnes breaking load



BRIDON

INTERNATIONAL

5.0 Complementary Role of Synthetic Fibre & Steel Moorings

5.1 Limitations

Synthetic fibre ropes offer the highest strength to weight ratio performance currently available.

However, as the fine fibre filaments are susceptible to damage through abrasion there are usage

limitations.

In a taut moored arrangement the lower end of the mooring cable is connected directly to an anchor

and will be embedded in the seabed and subject to considerable wear action. Therefore, currently

available fibre mooring materials are not suitable for the lower 150 - 200 metres of the mooring line

above the seabed. At the upper end similar constraints are apparent as fibre products are not

suitable for long term use with winches and sheaves. Furthermore, the potential for marine growth in

the first 50 -100 metres below the splash zone prohibits the use of fibre product in this section.

Although various coatings, filters and treatments are being investigated to protect the fibre, the

products currently available are typically suitable for use in the central taut segment away from the

seabed, mechanical handling and marine life.

Figure 6 — Taut Moored System



BRIDON

INTERNATIONAL

With the taut moored arrangement and the low weight synthetic cables, the introduction of steel into

the system serves the critical purpose of providing weight to maintain the tension in the overall

mooring line. Hence a requirement for cables with a low strength to weight ratio exists, contrary to the

physical requirements of a conventional catenary mooring system.

5.3 Fibre & Steel in Series

When connecting any non-similar components in series the effect of each on the others must be

carefully considered. Experience to date has suggested that the two critical factors in a combined

taut moored system are torsion and fatigue performance.

5.3.1 Tension-Torsion Fatigue

Six strand ropes will rotate when axially loaded. Fibre ropes having a very low torsional stiffness and

thus provides no resistance to the rotation generated by the six strand rope. The fibre rope

effectively acts as a low friction swivel. The fibre rope is largely unaffected by the imposed rotation.

However, the continued tension torsion fatigue loads have been proven to dramatically reduce the

life span of the six strand rope (Chaplin et al 1999). In conventional catenary systems six strand wire

rope is prevented from rotation by the inherent torsional stiffness of mooring chain, hence the

reduced torsion tension fatigue life is not apparent.

Six strand rope is now considered unsuitable for long term use connected in series with fibre ropes.

Torsionally balanced constructions such as spiral strand and chain, which do not generate rotation

when loaded, are recommended in this scenario.

5.3.2 Tension-Tension Fatigue

Due to the restoring forces originating from the elasticity of the polyester rather than the weight of

steel, taut moored systems are subject to higher tension fatigue loadings than a conventional

catenary system. Both fibre mooring ropes and spiral strand show excellent performance in tension-

tension fatigue loading. Standard mooring chain shows a much reduced performance in fatigue and

in order to achieve the necessary performance the chain must be oversized (Snell et al 1999). Where

minimising self weight is a critical driver, having to oversize the heavy chain components is not

desirable



BRIDON

INTERNATIONAL

6.0 Conclusions

Advances in mooring system design have developed mooring arrangements, using proven

technology steel components, which are both technically and economically suitable for 2000 metres

water depth. Recent developments in spiral strand design can now offer a further 10% reduction in

weight when compared to the current industry norm.

Fibre systems offer the highest strength to weight ratio allowing exploration into ultra deep locations.

In order to protect the sensitive fibre product against wear, the use of steel is still apparent in such

taut systems. Spiral strand offers the most cost effective, weight conscious and technically

advantageous solution to the connection between anchor and fibre rope.

Although the future for ultra deep mooring can be seen to be pursuing the trend for synthetic

solutions, the supporting role of steel cable is essential to its success. The continuing developments

in spiral strand strength will continue to assist engineering companies and operators with the

economical development of offshore prospects.

Steve Calverley

Project Sales Manager — Offshore

Bridon International Ltd

Carr Hill

Doncaster, DN4 8DG

UK

Tel: +44 (0)1302 382245

Fax: +44 (0)1302 382223

E-mail: [email protected]



BRIDON

INTERNATIONAL

References

Blair, Etheridge, Hall & Poranski (1995) Composite Catenary Anchor Leg Systems for Multiple Riser

Floating Production Vessels DOT Proceedings 1995

Firth, K.M. and Sefton, S.L. (1998) Technological Advances in Steel Mooring Cables: Finding

solutions for even greater water depths. FPS 98.

Sorrel, Fulton & Librino (1997) Installation of Deep Water Moorings . ASME Energy Week 97.

Chaplin, Rebel & Ridge (1999) Let s Not Twist Again. Offshore Engineer March 1999.

Snell, Ahilan & Versavel (1999) Reliability of mooring systems: application to polyester moorings.

31st OTC Conference May 1999 (Paper 10777).



BRIDON

INTERNATIONAL

Appendix 1: Bridon Xtreme High Strength Spiral Strand - Fatigue Performance

Two full scale fatigue tests have been carried out as described below,

Objective:

To establish the fatigue performance of Spiral strand comprising of high silicon chemistry final hot dip

galvanised (A class) 5.0mm / 5.3mm diameter wires — Bridon Xtreme Spiral Strand .

Procedure:

Test completed at DMT, Bochum procedure on their horizontal 6MN fatigue testing machine in

accordance with the Bridon procedure CP024.

Cable partially lubricated during manufacture (inner layers only) during sample preparation additional

light lubricant (Brilube 30) was sprayed onto the surface. Additional lubricant was sprayed during

sample testing.

Test sample length: 4 metres approx.

Terminations: Cylindrical dummy sockets.

Cable MBL: 1100 tonnes

Test 1: Load case, 30% mean load with –10% fluctuating load.

Test 2: Load case, 20% mean load with –10% fluctuating load.

Load frequency: 2 Hz.

Result:

Test 1: 384,650 cycles but the test was stopped whilst the sample could still support the upper load.

Test 2: 808,279 cycles achieved prior to the sample no longer being able to support the upper load.

Discussion:

Based on the API RP 2SK: NRM

= K (equation 6.10)

Where: N = Number of cycles

R = Tension range (double amplitude) to nominal breaking strength

M = Slope of T-N curve (5.05 for spiral strand)

K = intercept of T-N curve

Lm = ratio of mean load to MBL



BRIDON

INTERNATIONAL

Test 1 Test 2

Test conditions 30%–10% 20%–10%

Lm 0.3 0.2

K = 10(3.25-3.43Lm)

166 366

M 5.05 5.05

R 0.2 0.2

Expected N 562,220 1,239,595

Achieved N 384,650 808,279

The next lower band for fatigue assessment in accordance with API RP 2SK is the six / multi-strand

assessment.

The M and K parameters are amended for six strand wire rope giving predicted cycles to failure as

follows:

Test 1 Test 2


Lm 0.3 0.2

K = 10(3.20-2.79Lm)

231 438

M 4.09 4.09

R 0.2 0.2

Expected N 166,878 316,418

Achieved N 384,650 808,279

Hence, we can conclude the Xtreme spiral strand performs in excess of the six strand wire rope

assessment.

Therefore we have an upper and lower bound limit between which the Xtreme spiral strand can be

assessed.



BRIDON

INTERNATIONAL

Test 1 Test 2


Achieved N 384,650 808,279

R 0.2 0.2

M 5.05 5.05

Lm 0.3 0.2

Therefore:

K 114 238

Log K 2.055 2.378

Hence we can estimate a = 3.024

And b = 3.230

Therefore, for Xtreme, we can let: K = 10(3.024-3.23Lm)

For a point of comparison a typical fatigue assessment for a Gulf of Mexico Spar was completed

using the current spiral strand, six strand wire rope and the newly developed Xtreme M and K

parameters. The resultant design lives were as follows:

StandardSpiral Strand

XtremeSpiral Strand

Six StrandWire Rope

Fittings*

Life span 1.8 x 106

yrs 1.2 x 106

yrs 2.2 x 105

yrs 9.45 x 104

yrs

* Fittings are assessed as chain and hence remain unaltered for each of the above cases.

Conclusions and Recommendations

The Xtreme spiral strand fatigue performance is in excess of the fatigue performance of equivalent

breaking load six strand wire rope as defined by API RP 2SK.

For a typical Gulf of Mexico Spar assessment of the Xtreme spiral strand in accordance with the six

strand parameters suggests the terminations will remain as the limiting factor in fatigue life



BRIDON XTREME SPIRAL STRAND RANGEBRIDON

Appendix 2: Provisional data sheet INTERNATIONAL

BRIDON International Ltd Part of the FKI Group of Companies of 14

Nominal Diameter

Xtreme Spiral

Strand MBL

Increment over

SPR2 plus

Nominal

Weight/metre (kgs)

Nominal

Steel Area

Sheathing

Thickness

Axial

Stiffness

(in/mm) (kN) (kN) Unsheathed Sheathed submerged (mm2) (mm) (MN)

2.5" (65mm) 4680 11.4% 21.3 23.0 17.9 2562 6 423

2.625" (68mm) 4915 10.4% 22.4 24.2 18.7 2689 6 444

2.75" (70mm) 5355 10.4% 24.4 26.3 20.4 2901 8 479

2.875" (73mm) 5705 10.2% 26.1 28.0 21.8 3096 8 5113.00" (76mm) 6225 10.2% 28.4 30.4 23.8 3377 8 557

3.125" (79mm) 6785 11.4% 30.4 32.5 25.4 3614 8 578

3.25" (82mm) 7235 10.5% 33.0 35.1 27.5 3917 8 627

3.375" (86mm) 7930 10.3% 36.2 38.7 30.2 4300 8 688

3.50" (90mm) 8750 10.2% 39.9 42.9 33.4 4747 10 760

3.625" (92.5mm) 9265 10.4% 42.2 45.3 35.3 5020 10 803

3.75" (95.5mm) 9850 10.3% 44.9 48.1 37.5 5341 10 855

3.875" (98mm) 10445 10.4% 47.6 51.0 39.8 5656 10 905

4.00" (102mm) 11255 9.6% 51.6 55.3 43.1 6139 11 982

4.125" (105.5mm) 11935 9.8% 54.7 58.4 45.7 6499 11 1040

4.25" (108mm) 12560 9.9% 57.5 61.3 48.0 6834 11 1093

4.375" (111.5mm) 13345 10.0% 61.0 65.0 51.0 7254 11 1161

4.50" (114mm) 13995 9.5% 64.2 68.3 53.6 7640 11 1222

4.625" (118mm) 14940 9.9% 68.4 72.6 57.1 8130 11 1280

4.75" (121.5mm) 15800 10.0% 72.2 76.5 59.7 8589 11 1353

4.875" (124mm) 16580 10.0% 75.9 80.3 63.4 9014 11 1420

5.00" (127mm) 17330 10.2% 79.1 83.6 66.0 9403 11 1481

5.125" (131mm) 18500 10.3% 83.3 87.9 69.6 9899 11 1534

5.25" (133mm) 18970 10.5% 86.8 91.5 72.4 10314 11 1599

5.375" (137.5mm) 20080 9.9% 92.5 97.3 77.2 10991 11 1704

5.50" (141mm) 21080 9.9% 97.5 102.4 81.5 11609 11 1799

5.625" (144mm) 21795 9.7% 101.3 106.3 84.6 12034 11 1865

5.75" (146.5mm) 22430 9.6% 105.1 110.2 87.7 12515 11 1940

5.875" (147.5mm) 22880 9.5% 107.2 112.4 89.5 12718 11 1971

6.00" (153mm) 24165 9.5% 114.5 119.7 95.5 13616 11 2110

Nominal

Weight/metre (kgs)

mooring bridon

Documents

mooring solution

taut mooring

mooring system loads

mooring system design

steel mooring line segments

steel system

mooring cable solutionsauthor

wire performance mooring